1. Field of the Invention
The present invention generally relates to a demodulation apparatus and a demodulation method for demodulating a received signal with digital modulation and particularly relates to a demodulation apparatus and a demodulation method for selectively changing demodulation modes to use depending on the quality of a received signal or according to a control signal.
2. Description of Related Art
Implementation of Wireless Personal Area Network (WPAN) to enable small scale wireless communication between home electric appliances or equipment for transmitting digital contents (e.g. digital camera) has been recently discussed in IEEE 802.15 Working Group for WPAN TG3a (Task Group 3a WAPAN at High rate PHY) or the like. In the new data communication technique over WPAN, there is required an increase in information transmission speed and reliability for application to transmission of multimedia information. There are also required measures against noise, interference and so on that are caused by transmission over other WPAN devices or the like.
Orthogonal Frequency Division Multiplexing (OFDM) that has high frequency efficiency and high resistance to multipath and that is expected to be applied to WPAN is one type of multicarrier transmission methods. The frequencies of a plurality of subcarriers (sine waves) that form an OFDM symbol are set so that the subcarriers are orthogonal to each other within one symbol duration. The modulation to generate an OFDM signal is performed by Inversed Fast Fourier Transform (IFFT) on the amplitude and phase of each subcarrier using the condition that a plurality of subcarriers are orthogonal on a frequency axis. On the other hand, the demodulation is performed by Fast Fourier Transform (FFT). Further, in OFDM, a guard interval is inserted into a symbol duration to avoid inter-symbol interference.
There are proposed a variety of subcarrier modulation modes including Quadrature Phase Shift Keying (QPSK) and 16 Quadrature Amplitude Modulation (16-QAM). To increase a transmission speed, it is effective to increase the number of bits per symbol. However, the increase in the number of bits by the use of multivalued bits results in a narrower interval between signal points, which reduces resistance to fading, interference wave, noise and so on.
In order to address this concern, adaptive modulation that adapts its modulation mode in accordance with a transmission speed or radio wave conditions is in practical use. Under strong radio wave conditions, this modulation method performs high-speed transmission using a high efficiency modulation mode such as 16-QAM modulation. Under weak radio wave conditions, on the other hand, the method performs signal transmission using a modulation mode that is highly resistant to fading, interference wave and noise, such as QPSK modulation. In this case, it is necessary to change both of the modulation mode used for modulation of a transmitting signal at a transmitting end and the modulation mode used for demodulation of a received signal at a receiving end.
An example of a demodulator using the adaptive modulation method is disclosed in Japanese Unexamined Patent Application Publication No. 09-275426 (FIG. 1). The demodulator selects a circuit to be used for demodulation by switching signal paths with a selector so as to demodulate a QAM modulation signal when an input signal is a QAM modulation signal and demodulate a QPSK modulation signal when an input signal is a QPSK modulation signal. This achieves an integral-type demodulator that is capable of demodulating two kinds of modulation signals in one system.
In the multicarrier transmission methods such as OFDM, the effects of fading, interference wave and noise on each subcarrier are not uniform. Even if the quality of one subcarrier in a plurality of subcarriers significantly deteriorates due to noise originating in a transmission path or the like, the quality of another subcarrier whose frequency is separated from the frequency of the one subcarrier can remain suitable.
As a technique for reducing the effects of frequency selective fading, interference wave, noise and so on using the above characteristics of the multicarrier transmission methods such as OFDM, a frequency diversity technique that transmits the same data through a plurality of subcarriers is known. This technique is described, for example, in Japanese Unexamined Patent Application Publication No. 2000-101496 and Makoto ITAMI, “A Study on Bidirectional High-Speed Transmission by OFDM”, Research Report of the Telecommunications Advancement Foundation of Japan, 2001, No. 16, p410-p418.
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The frequency diversity transmission system that is disclosed in Japanese Unexamined Patent Application Publication No. 2000-101496 is a multicarrier transmission system that includes a transmitter and a receiver to implement frequency diversity. The transmitter simultaneously transmits the same information through two subcarriers having different frequencies. The receiver includes a FFT processor that demodulates two subcarriers transmitted from the transmitter, two data decision circuits that compare data of the channel signals that are obtained by demodulating the two subcarriers, a selector that selects a decision value output from the two data decision circuits and outputs demodulation data, and a decision circuit that compares the decision values output from the two data decision circuits and outputs a selection signal to the selector so as to select the one with a better line quality (i.e. with a low error rate) as demodulation data.
ITAMI discloses an OFDM system to implement frequency diversity. In this OFDM system, a transmitting end transmits the same symbol through a plurality of carriers and a receiving end performs maximum ratio combining thereon for diversity reception.
An object of the invention disclosed in Japanese Unexamined Patent Application Publication No. 09-275426 is to use a common circuit for demodulation to thereby suppress an increase in circuit size. This patent document thus contains no disclosure regarding a technique that a demodulator receives a plurality of transmission subcarriers and, even if the quality of several subcarriers deteriorates due to noise or the like, performs demodulation in accordance with the signal quality to avoid deterioration in the quality of a modulation signal.
A conventional multicarrier transmission using frequency diversity cannot switch demodulation modes used in a receiving end unless switching modulation modes for subcarriers used in a transmitting end or applying different modulation modes to a plurality of subcarriers. Therefore, it requires a large circuit size due to redundancy in a modulator site in a transmitting end, a complicated control mechanism due to the need for transmitting control information to use the same modulation/demodulation mode in a transmitting end and a receiving end, and so on.
As described in the foregoing, the present invention has recognized that a conventional multicarrier transmission using frequency diversity has a drawback that it cannot switch demodulation modes in a receiving end unless switching modulation modes for subcarriers in a transmitting end or applying different modulation modes to a plurality of subcarriers.